4 Month Old Who Crys Fusses in sleep..night Terrors?

When my daughter had nighterrors at 9 months it was from being over tired! If they skip a nap or it's cut short for whatever reason then it would happen at night exactly what your describing.
So maybe add another nap or let naps go longer
Play around with sleeping schedules to see what fits your child the best.
I kno how horrible it feels to watch and not be able to soothe your bby.

Maybe have him checked out to make sure he doeant have an ear infection or any other things mj below me stated.

I know when my DD had ear infection she would do all those things because she was uncomfortable and couldn't settle.

My guess is teething...

Well it's an old wives tale where i live that until they can speak , babies can see angels and at times they play at times they tease the kids ad they cry :)
Another reason could be stomach cramps or infection of the ear or maybe the little nose is blocked...
Have you discussed it with the doc ? A doc can help you via Thorough checkup..
My daughter who is 9 months old used to get scared in her dreams and it would take quite an effort to calm her down... from distracting her to singing poems :)

Talk to your pediatrician about it.
Video an episode so you can show it to him.
I Google-d it and it seems night terrors can start at this age.
One mother wrote:
"My ped. told me to put him to bed, and then wake him (not totally awake), but enough to make him open and close his eyes, about 30 minutes after he has fallen asleep. We would wake him at 30 minutes, and then 10 minutes after that and 10 minutes after that (just to be sure.). Most nights it worked, and he had them nightly as well. By doing this somehow you get him to the REM stage quicker, which the NTs happen in the NREM stage prior to that."

Any signs of reflux? Nursing or formula? Dairy sensitive? Just a few thoughts from my tired brain. How are naps?